As great as the Anthony Joshua-Wladimir Klitschko heavyweight world title fight was on April 29, it got off to a rather slow start.

For the first four rounds, Joshua, defending his title and aiming to claim a vacant belt to go with it, and former longtime unified champion Klitschko were careful. Understandably, each was wary of the other because both possess explosive power capable of knocking out anyone with one shot.

The British-record crowd of 90,000 was waiting for a real fight to break out in the hugely anticipated showdown at London's Wembley Stadium and the fans ultimately got what they came for in a classic, which Joshua wound up winning by 11th-round knockout. But the excitement wasn't unleashed until the dramatic fifth round, which set the tone for rest of a great heavyweight title fight and earned 2017 ESPN.com round of the year honors.

Joshua stormed to Klitschko to open the round and hurt him with a series of punches that landed cleanly and cut him under the left eye in the first 20 seconds. A few seconds later, Joshua landed another flurry, including a clean right hand, a left and an uppercut. Klitschko was hurt and went down, eventually falling over face first.

He rose quickly and took the mandatory eight-count from referee David Fields but didn't look good. Joshua went right after him in an effort to finish the fight then and there -- and he came close. He nailed Klitschko with a solid left hook and continued to bomb away, also cutting him over the left eye.

The end was near, or so it seemed to everyone but Klitschko, whose chin had betrayed him in the past. The proud former champ sucked it up and went for broke. If he was going to get knocked out, he was going to go down swinging.

Joshua had expended a tremendous amount of energy and suddenly a resurgent Klitschko began to land powerful punches. He wobbled Joshua with a nasty left hook to the head and had turned the tables with the kind of shot that would have KO'd most opponents. With a minute left it was now Joshua in deep trouble, backing into the ropes and taking deep breaths.

Klitschko bashed Joshua with a right hand and it looked as if he were on the verge of rallying for a knockout to reclaim the title.

Blow-by-blow announcer Mauro Ranallo of Showtime, which also televised the fight in the United States, summed up the round as it neared the end. "Terrific stuff here in the fifth frame between Joshua and Klitschko," Ranallo roared. "Good start by Joshua, better finish by 'Dr. Steelhammer.' "

Klitschko was getting the better of the action in the final few seconds, but Joshua landed a couple of shots as they traded on the inside.

"What a comeback by Klitschko in this round no matter what happens," HBO analyst Max Kellerman said. "That's a classic round in heavyweight championship action!"

Other scorchers

Gennady Golovkin, left, and Canelo Alvarez finished their middleweight title fight in September with a bang. Ethan Miller/Getty Images

The middleweight championship showdown lived up to the massive hype and closed with a bang with a fireworks-filled final round. Canelo and GGG delivered a toe-to-toe round that had the crowd on its feet. Alvarez started fast, stinging GGG with a right hand. He had a huge first minute as he ripped Golovkin with hard combinations. But GGG took them all and then backed up Alvarez with a powerful right hand and came on strong. They finished the fight banging away at each other as the sold-out crowd went wild. "Fire and fury in Las Vegas! Gennady Golovkin and Canelo Alvarez firing away down the stretch," HBO's Lampley said in the final frenzied moments of a fantastic fight.

3. James DeGale-Badou Jack, Jan. 14 at Barclays Center, Brooklyn, New York (12th):

It had been a tremendous battle as Jack and DeGale met in an attempt to unify super middleweight titles and the all-out action continued in the final round. With the fight razor close, Jack, who had been down in the opening round, salvaged a draw when he dropped DeGale to his backside with a clean right hand to the chin and nearly stopped him in a dramatic round. Jack battered DeGale following the knockdown. He took some enormous shots but managed to stay on his feet even though his mouthpiece got punched out. As much trouble as DeGale was in he rallied in the final seconds to land clean shots, including a big right hand. Showtime's Ranallo loved what he saw: "Final minute of the 12th! Big drama in the Big Apple!"

In the midst of a heavyweight slugfest, Ugonoh appeared on the verge of dropping Breazeale 40 seconds into the third round when he lashed Breazeale with two rights and a left. He was having a huge round but as he went to fire another punch, Breazeale countered with a right hand that dropped Ugonoh hard. Ugonoh stormed back, forcing Breazeale to grab on to him and tackle him to the mat. Ugonoh continued to dish out punishment, but Breazeale found the energy to turn the tables in the final seconds.

Hurd, defending his junior middleweight belt for the first time, and former titleholder Trout traded nonstop. They landed head and body shots back and forth, though the heavier hitting Hurd did major damage. He badly hurt Trout with a right hand that nearly dropped him with 30 seconds to go before Trout came back in the final seconds of the action-packed round.

Lightweight Molina, down hard in the second round, rebounded to floor Redkach. But the round began with Redkach rocking Molina with a left hand. Molina shook it off, closed the distance and dropped Redkach hard with a right hand with about a minute to go. Redkach got a few extra seconds to recover while his mouthpiece was replaced and then swung for the fences as they brawled toe-to-toe for the rest of the round. "You have gotta be kidding me! What intestinal fortitude by these two warriors," Premier Boxing Champions announcer Ray Flores said as they slammed each other with clean punches in the final seconds.

The expected junior lightweight slugfest was in full swing by the time they got to the eighth round of what was an even fight to that point. As they punished each other, Roman scored his second knockdown of the bout with a minute left when he nailed Salido with a left hook in the midst of a combination. Salido took his time to get up and looked utterly dejected before Roman continued to assault him to the head and body. Salido was hurt and looked done, but the old warrior dug deep yet again and had Roman in retreat while he landed big shots in the waning moments.

Oscar Valdez, left, and Genesis Servania gave fans a tremendous 12th round during their featherweight title fight in September. Photo provided by Mikey Williams/Top Rank

Featherweight titlist Valdez and Servania punctuated their battle with a high-energy final round by trading with abandon as Valdez's hometown crowd cheered "Oscar! Oscar!" The action was excellent in the first minute, prompting ESPN's Joe Tessitore to exclaim, "Oh, this is gonna be sensational stuff down the stretch!" Servania got in an uppercut and a right hand that knocked Valdez back, but Valdez ripped him with 1-2 combinations. They finished the fight in a heated exchange to conclude a tremendous battle.

The unbeaten middleweight prospects put on quite a show, especially in the third round in which they were both in trouble. Aleem blasted Khytrov with a left hook that dropped him hard with a minute left. Khytrov was in bad shape but rallied, badly hurting Aleem and nearly stopping him with a series of shots late in the round. Aleem then got in a few blows, but as the round was ending Khytrov backed him into a corner and had him out on his feet.

These junior welterweights took their fight from interesting to thrilling in the third round when both were knocked down. Ramos was dominating when Baranchyk clobbered him with an overhand right to drop him with 40 seconds left. But seconds before the bell, Ramos dropped Baranchyk in exaggerated fashion with a left hand to end the wild round.